American oil trains spilled crude oil more often in 2014 than in any year since the federal government began collecting data on such incidents in 1975, an NBC News analysis shows.

The record number of spills sparked a fireball in Virginia, polluted groundwater in Colorado, and destroyed a building in Pennsylvania, causing at least $5 million in damages and the loss of 57,000 gallons of crude oil.

By volume, that's dramatically less crude than trains spilled in 2013, when major derailments in Alabama and North Dakota leached a record 1.4 million gallons -- more than was lost in the prior 40 years combined. But by frequency of spills, 2014 set a new high with 141 "unintentional releases," according to data from the federal Pipeline and Hazardous Materials Safety Administration (PHMSA). By comparison, between 1975 and 2012, U.S. railroads averaged just 25 spills a year.

The vast majority of the incidents occurred while the trains were "in transit," in the language of regulators, rumbling along a network of tracks that pass by homes and through downtowns. They included three major derailments and seven incidents classified as "serious" because they involved a fire, evacuation or spill of more than 120 gallons. That's up from five serious incidents in 2013, the data shows.

"They've got accidents waiting to happen," said Larry Mann, the principal author of the landmark Federal Railroad Safety Act of 1970. "Back in 1991 I said, 'One day a community is going to get wiped out by a freight train. Well, in 2013 that happened and unless something changes it's going to happen again."

Mann was referring to the Lac-Mégantic disaster, a deadly derailment in Quebec just miles from the Maine border. A 72-car oil train rolled downhill and exploded on July 6, 2013, killing 47 people and destroying most of the town.

In the months that followed American regulators convened a series of emergency sessions. They promised sweeping new safeguards related to tank car design, train speed, route and crew size. To date none of those rules have been finalized.

On January 15 the Department of Transportation missed a deadline set by Congress for final rules related to tank cars, which have a decades-long history of leaks, punctures, and catastrophic failure. The rules are being worked on by PHMSA and the Federal Railroad Administration (FRA).

In response to questions from NBC News, PHMSA declined to explain the delay in new rules but it defended the relative safety of oil-by-rail. "More crude is being transported across the country than in any time in our history, and we are aggressively developing new safety standards to keep communities safe," PHMSA spokesperson Susan Lagana said in a statement.

"Last year, over 87,000 tank cars were in use transporting crude oil, and 141 rail crude oil releases were reported," she continued. "The amount of crude oil released in these spills was less than the capacity of two tank cars."

The FRA declined a request for comment. It did, however, provide data that suggests the railroads are getting better overall at transporting hazardous material. Between 2004 and 2014, for example, the number of collisions and derailments involving trains carrying hazardous material fell by more than half, from 31 to 13, according to the data.

Ed Greenberg, a spokesperson for the Association of American Railroads (AAR), the industry's principal trade group, said the railroads themselves support stronger tank cars. The oil industry actually owns most of the cars used to transport its product, he said. That has complicated the rule-making process and set off a debate over which industry should cover the cost of an upgrade.

Greenberg also sharply disagreed with the idea that oil-by-rail was getting more dangerous. With 40 times more oil being hauled along U.S. rail lines in 2015 than in 2005, he acknowledges that the raw number of incidents has increased. But he argues that the railroads have never been safer overall.

"Railroads have dramatically improved their safety over the last three decades, with the 2014 train accident rate trending at being the lowest ever," he told NBC News, citing multi-billion-dollar investments in new cars, tracks, and workers.

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Last year, he added, 99.97 percent of all hazardous material on the rails reached its destination without incident. Of the 141 oil spills included in the federal data, meanwhile, the AAR calculates that fewer than 10 involved the loss of more than a barrel of oil.

But critics say that's little comfort to the estimated 25 million Americans who within the one-mile evacuation zone that the US Department of Transportation recommends in the event of an oil train-derailment.

"Moving oil from one place to another is always risky, and even a single spill has the potential to harm land and marine ecosystems for good," said Karthik Ganapathy, communications manager for 350.org, an environmental group that has helped organize protests against oil by rail. "These new data confirm what we've known to be true all along—oil-by-rail is incredibly dangerous."